It is well known that a spacecraft includes thrusters for station-keeping in orbit and that these thrusters are supplied from a finite reservoir.
In order to maximise the life of a spacecraft it is important to be able to determine very accurately the mass of propellant remaining at any given instant in time and particularly at the end of life when a small quantity of propellant must be reserved for the final graveyard burn. Traditionally the measurement of the remaining propellant has been accomplished by integrating the predicted propellant consumption for all the liquid propellant gauging and thruster firings over the spacecraft life with check calculations being performed from the tank pressure and temperature telemetry. This method suffers from the fact that the greatest accuracy is found at beginning of life, whilst at end of life, when a high accuracy is most needed, relatively low accuracy is achieved.
Also, of course, what appears as a low percentage error when the vessel is full becomes a much larger percentage error when the tank is nearly empty. In practice this means that the spacecraft often will contain more propellant than is required for its service life and it will still have this usable excess even after the final graveyard burn. Thus an improvement in the accuracy of measurement of the mass of the propellant fluid may provide an increase in the service life and/or a decrease in the mass of fluid loaded at launch. In an earlier proposal, the temperature rise over a fixed period immediately after a tank heater is switched on is detected and related in a linear fashion to the thermal capacity of the system.